Cigarettes typically include a tobacco rod and a filter. The filter is located at the mouth end of the cigarette, between the smoker and the tobacco rod. The filter modifies the tobacco smoke (mainstream smoke) drawn through it. Filters may be intended to reduce or alter various smoke constituents, including particulate matter and/or vapour phase matter.
Multi-segment filters are becoming more common. For example, in one arrangement a filter includes a section comprising activated carbon, which has good filtration properties. The filter may include another section between the activated carbon and the mouth-end of the cigarette. This helps to ensure that the activated carbon does not enter the mouth of the user.
There are many known techniques for altering filter behaviour. One technique is to include ventilation holes in the outside of the filter. This allows air to be drawn into the filter, and so dilutes the cigarette smoke inhaled by a user. Another known technique is to provide grooves in a filter, often along the outside of the filter. These grooves can be used to control flow properties through the filter. Examples of grooves and/or ventilation holes can be found in: WO 03/051144; GB 2150809; GB 2150412; GB 2118819; GB 2089641; GB 2088692; GB 2088193; GB 2088191; GB 1585862; GB 1308661; EP 047969; U.S. Pat. No. 4,527,573; U.S. Pat. No. 4,527,572; U.S. Pat. No. 4,256,122; U.S. Pat. No. 4,135,523; U.S. Pat. No. 3,768,489; and U.S. Pat. No. 3,752,165.
It remains desirable to provide improved filtration properties for a multi-segmented filter.